Conventionally, a method for detecting the origin of a lens unit is proposed in which when the lens unit is driven by a motor, a photo-interruption member attached to the lens unit and a photosensor are used so as to monitor an output level of the photosensor at the time when the photo-interruption member transverses the photosensor (Patent Document 1, for example).
A conventional lens driving apparatus is described below, with reference to FIG. 58. FIG. 58 includes a schematic diagram and a block diagram of an exemplary conventional lens driving apparatus. In an imaging device 75, an image of a subject captured through a fixed lens 72 fixed to a barrel 71, a zoom lens 73 and a focus lens 74 is converted into an electric signal. Based on the electric signal output from the imaging device 74, a signal processing unit 82 generates image data and contrast information for performing focus adjustment.
When the power of a main unit is turned on, a system control unit 81 outputs an instruction to a focus motor control unit 80 so as to drive the focus lens 74 to the side of the imaging device 75. Based on the information concerning a moving direction and a moving step from the focus motor control unit 80, a focus motor driving unit 83 outputs a driving signal to a motor 79 so that a desired rotation direction and such a rotation movement amount can be obtained. The focus motor control unit 80 also receives a rotation position of a zoom ring 76 that is detected by a zoom ring position detection unit 84.
When the focus lens 74 reaches the proximity of the position indicated by the dotted line of FIG. 58, a photosensor 78 is interrupted by a photo-interruption member 77, so that an output signal level of the photosensor 78 changes. When this output signal level exceeds a certain threshold level (or falls below a threshold value in some circuit configurations), a counter provided for the focus motor control unit 80 beforehand is reset so as to detect an absolute position of the focus lens 74. Concurrently with this, positional information of the focus lens 74 for focus adjustment is output to the system control unit 81.
In this way, the absolute position of the focus lens 74 and a positional relationship with the zoom lens 73 are controlled, whereby various applications can be considered. For example, even in the case of performing a zooming operation, the position of the focus lens 74 can be controlled while maintaining the focusing condition, a retracting speed in an auto-focus function can be increased and a distance from a subject can be estimated from the absolute position information of the focus lens 74.
Meanwhile, in the case where a focus lens in an interchangeable lens type imaging device is driven by a motor, an example of a lens barrel equipped with a motor that shifts a focus lens, a driving circuit that drives the motor and a microcomputer that controls a position of the motor is known conventionally.
Such a conventional imaging apparatus is described below, with reference to FIG. 59. FIG. 59 includes a schematic diagram and a block diagram of an exemplary conventional imaging apparatus. FIG. 59 shows an example of an interchangeable lens type imaging apparatus capable of detaching a lens barrel 88 from a camera main body 89, where the detaching can be conducted at a junction part (not illustrated) of a signal line between a motor control unit 86 and a system control unit 81.
An imaging device 75 converts an image of a subject captured through a fixed lens group 72 and 85 fixed to the lens barrel 88 and a focus lens 74 into an electric signal. Based on the electric signal output from the imaging device 75, a signal processing unit 82 generates image data and contrast information for performing focus adjustment.
When the power of a camera main body 89 is turned on, the system control unit 81 outputs an instruction to the motor control unit 86 so as to drive the focus lens 74 to the side of the imaging device 75. The motor control unit 86 reads out information indicating a relationship between a subject distance and a focus lens position that is stored in a storage device 85. Based on the information concerning a moving direction and a moving step from the motor control unit 86, a motor driving unit 87 outputs a driving signal to a motor 79 so that a desired rotation direction and such a rotation movement amount can be obtained.
When the focus lens 74 reaches the proximity of the position indicated by the dotted line of FIG. 59, a photosensor 78 is interrupted by a photo-interruption member 77, so that an output signal level of the photosensor 78 changes. When this output signal level exceeds a certain threshold level (or falls below a threshold value in some circuit configurations), a counter provided for the motor control unit 86 beforehand is reset so as to detect an absolute position of the focus lens 74.
With the use of the thus detected absolute position of the focus lens 74, a retracting speed in an auto-focus function can be increased, and a distance from a subject can be estimated from the absolute position information of the focus lens 74. Further, by using information on focus deviation that is output from the system control unit 81 and information on the focus lens position read out from the storage device 85, the motor control unit 86 can control the focus lens position.
The technology described in the following Patent Document 2 also relates to interchangeable lens type audiovisual equipment. A control unit 119 provided in a lens unit 127 refers to not only lens cam data 120 stored beforehand inside a lens microcomputer but also an AF evaluation signal sent from a main body microcomputer 114, whereby a scaling operation can be conducted while keeping a position where an AF evaluation value is the maximum.
Further, Patent Document 3 describes a mechanism for detecting the origin of a lens unit. FIG. 60 is a schematic perspective view of a main portion of another exemplary conventional imaging apparatus. In FIG. 60, numeral 91 denotes a reset switch as a reference position (reset position) detector that is fixed to a stationary member (not illustrated).
The reset switch 91 has a U-shaped main body as illustrated, and an upper horizontal strip portion 91a (hereinafter called “top plate portion”) and a lower horizontal strip portion 91b (hereinafter called “bottom plate portion”) of the main body are arranged parallel to an optical axis of an optical system described later. A detection target plate protruding from a lens holder described later can enter in a space between the top plate portion 91a and the bottom plate portion 91b. 
A photo-transmission element is attached to a lower face of the top plate portion 91a, and a photo-reception element is attached to an upper face of the bottom plate portion 91b so as to be opposed to the photo-transmission element. The photo-reception element and the photo-transmission element make up a photo-interrupter, where the photo-reception element is connected electrically with a controller 90 on a electronic circuit board via an electric wiring W1.
Numeral 92 denotes a focus lens holder that holds a focus lens group. A feed screw engaging strip (or female helicoid member) 92b provided with a screw hole threadably engaged with a feed screw 98 is provided around the holder 92. Further, a sleeve-shaped sliding unit 92c axially slidably fitted to a first guide bar 96 and a projection strip 92d with a U-shaped groove axially slidably fitted to a second guide bar 97 are provided. Moreover, a detection target plate 92a capable of entering into the space between the top plate portion 91a and the bottom plate portion 91b of the reset switch 91 is provided.
The feed screw 98 extends parallel to the optical axis of the lens and is fixed to a shaft of a stepping motor 94 for driving the focus lens. The first guide bar 96 and the second guide bar 97 extend parallel to the optical axis of the lens and are fixed to a stationary member (not illustrated).
Numeral 93 denotes a zoom lens holder that holds a zoom lens group, and is disposed coaxially with and at a predetermined interval from the focusing lens holder 92. A feed screw engaging strip (or female helicoid member) 93b provided with a screw hole threadably engaged with a feed screw 99 is provided around the zoom lens holder 93.
Further, sleeve-shaped sliding portion 93c axially slidably fitted to the first guide bar 96 and a projection strip with 93d with a U-shaped groove axially slidably fitted to the second guide bar 97 are provided. Moreover, a detection target plate 93a capable of entering into the space between the top plate portion 91a and the bottom plate portion 91b of the reset switch 91 is provided. The feed screw 99 extends parallel to the optical axis of the lens and is fixed to a shaft of a stepping motor 95 for driving the zoom lens.
The stepping motor 94 is connected to the controller 90 via a wiring W2 and the stepping motor 95 is connected to the controller 90 via a wiring W3.
In the thus configured conventional imaging apparatus, when the power is supplied by a power supply switch (not illustrated), firstly the stepping motor 95 begins to rotate, so that the feed screw 99 rotates. Thereby, the zoom lens holder 93 is shifted toward the front end of the screw 99 along the feed screw 99.
Then, when the detection target plate 93a enters into the space between the top plate portion 91a and the bottom plate portion 91b of the reset switch 91, light bundle from the photo-transmission element as a photo-reflector is intercepted by the detection target plate 93a, and in response to this, the controller 90 drives the stepping motor 95 while counting the step number, so as to shift the zoom lens holder 93 to the initial set position.
Next, the stepping motor 94 rotates so that the focus lens holder 92 is shifted toward the front end of the feed screw 98. When the detection target plate 92a enters into the space between the top plate portion 91a and the bottom plate portion 91b of the reset switch 91, thus intercepting light from the photo-transmission element, the controller 90 accordingly drives this stepping motor 94 while counting the step number, so as to shift the zoom lens holder 92 to the initial set position.
In this way, in this conventional apparatus, the detection of the reset positions of the zoom lens and the focus lens, i.e., the detection of the origins can be accomplished with the detection target plates provided for the respective lens holders and one reset switch common to the two lenses.
Patent Document 4 discloses a focus adjustment apparatus for camera in which a lens group and a stop are driven by a pulse (stepping) motor that is pulse-driven in a one to two phase excitation manner. The focus adjustment apparatus for camera described in Patent Document 4 has three pulse motors including a pulse motor M1 for stop, a motor M2 for focus adjustment and a zoom motor M3. The origins of the pulse motor M1 for stop and the motor M2 for focus adjustment are detected using a photosensor that is provided separately from a lens group and wings for stop that are members to be driven. As for the zoom motor M3, the absolute position of the lens group is detected by a volume (variable resistor), and therefore the origin therefor is not detected.
Patent Document 5 discloses a lens driving apparatus having a stepping motor. In the lens driving apparatus described in Patent Document 5, the origin is detected by shifting a lens as a member to be driven at a limiting position that is regulated mechanically and then reverse-driving the lens from the limiting position by a predetermined moving amount. According to Patent Document 5, such control allows the origin to be detected with high precision.
However, in the conventional lens driving apparatus like FIG. 58, the positional relationship between the photo-interruption member attached to the lens unit and the photosensor differs in absolute position for each detection operation because of errors in looseness of the lens unit in the driving direction and variations in mechanism and electrical properties due to temperature and humidity changes in the operation environment, thus making it difficult to obtain suitable performance for realizing a high quality image, etc.
Meanwhile, there is proposed a method in which two photosensors having different variation sensitivities in output level with respect to the shift amount of a photo-interruption member when the photo-interruption member traverses the photosensors are used. An output of the photosensor having a larger variation sensitivity is set at a start signal and the origin is detected from an output of the photosensor having a smaller variation sensitivity. This method is advantageous for enhancing the detection accuracy of the absolute position, but is disadvantageous in terms of compact size and cost.
Further, in the conventional imaging apparatus like FIG. 59, a large scale of microcomputer is required for controlling both of the lens barrel side and the camera main body side. Therefore, in an interchangeable lens type imaging apparatus, it is difficult to realize a compact lens barrel and a low cost. Further, there is a variation in focus position because of errors of variations in mechanism and electrical properties due to a temperature and humidity change in the operation environment of the lens barrel, thus making it difficult to obtain sufficient performance.
Further, according to the origin detection method in the conventional imaging apparatus like FIG. 60, the movement of the photo-interruption members is detected with a common photosensor so as to detect the origins. However, the photosensor is disposed between both lens units and that is located around the units. Therefore, the outer dimensions of the lens units increase, thus increasing the lens barrel in size.
Moreover, when storing the lens units, the lens units have to be closer to each other. However, in order to avoid the contact between the respective photo-interruption members at this time, the outer dimensions of the photosensor should be increased. This becomes a factor of limiting downsizing in the optical axis direction and its orthogonal direction, which means an obstacle to downsizing of the lens barrel.
Further, in the origin detection method shown in FIG. 60, the following problems occur in the case of abnormal completion. The abnormal completion refers to the completion caused by a decrease in voltage because the battery for supplying to the imaging device becomes exhausted or careless detachment of a connection terminal to an external power supply during an operation using the external power supply, for example. In this case, when the power of the imaging device is turned on next, a process for detecting the origin of the zoom lens unit will be normally performed. In this case, if the light of the photosensor is interrupted by the photo-interruption member of the focus lens unit because of a decrease in voltage, the process for detecting the origin cannot be performed normally and a malfunction will occur. In this way, the conventional example having a photosensor used common to the zoom lens unit and the focus lens unit has several problems.
Further, the focus adjustment apparatus described in Patent Document 4 requires the configuration such as a photosensor separately provided to detect the origin of the stepping motor, thus having a problem of being incapable of achieving the downsizing of the imaging apparatus.
Further, the lens driving apparatus described in Patent Document 5 detects the origin by shifting a member to be driven to the limiting position that is regulated mechanically. Therefore, there is a problem that an error occurs when the shifting amount from the origin is specified from the pulse number applied to the stepping motor. This results from, when the member to be driven is allowed to contact with the limiting position regulated mechanically, the member to be driven will receive a magnetic force applied to a rotor magnet in a different direction depending on the exciting position corresponding to the limiting position, and therefore the member to be driven is driven in two ways depending on the timing when the origin is set, i.e., in the direction toward the limiting position and in the direction away from the limiting position.    Patent document 1: JP H06(1994)-174999 A    Patent document 2: JP H09(1997)-23366 A    Patent document 3: JP H04(1992)-184309 A    Patent document 4: JP H10(1998)-224680 A    Patent document 5: JP H08(1996)-76005 A